Most photographic film now used is 35 mm format film. Cameras have been developed to allow film of this format to be easily loaded by placing the film cassette, in which the film is stored, into an opening in the back of the camera and then shutting the back of the camera. As a free end or tongue of the film extends externally to the film cassette, this is used to automatically advance the film in the camera for picture taking. Once all the pictures have been taken, the film is rewound into the film cassette for removal from the camera for processing. In order for this to work, the free end or tongue of the film must extend externally to the cassette so that it can be pulled on to the take-up spool in the camera as the film is first loaded.
Photographic film tends to be processed in a single strip once the film has been removed from its cassette. Strips of negative film are processed by transporting them, either as a single individual strip or as a continuous length comprising two or more strips of shorter lengths, through a series of processing solutions in various tanks in the processing apparatus.
In known processing apparatus, the film strip is pulled through tanks containing the processing solutions either by a leader which is attached to the leading edge of the film strip, or by moving a rack or spiral containing the film strip from tank to tank. Individual film strips may be pre-spliced into a long reel with a leader card at the front end, clipped to a rack, or fed into a spiral.
Where the film strip is attached to a leader, it is unloaded from the cassette and attached to the leader in a manual operation. The leader is then fed into the processing apparatus so that the film can be processed as it is transported through the apparatus.
Operations of splicing the film strips together or attaching the leader to the strip need to be carried out in darkroom conditions due to the sensitive nature of the film.
However, in some processing apparatus, there is a minimum length of film strip which can be processed. As a result, several film strips need to be spliced together prior to processing. This may be time-consuming as each strip will need to be measured to ensure that the minimum processing length is present in the spliced strip prior to processing.
In order to overcome the problems mentioned above, a loading device for a photographic processing apparatus in which the film strip is automatically unloaded from its cassette, its length checked, and then fed into the processor if the film strip exceeds the minimum length under the control of the process computer is provided. In this arrangement, a motor is used to provide drive to a pair of rollers to remove the film from its cassette. However, it is known to use a motor to advance and rewind film in a camera, the motor being either integral with the camera or a separate attachment. One such arrangement in which a separated attachment is used is disclosed in U.S. Pat. No. 4,118,659.
U.S. Pat. No. 4,322,149 discloses a device in which a motor is used to wind up a film. A wind-up signal is produced when an exposure has been completed which drives the wind-up motor. The torque produced by the motor is detected and an increase in torque corresponding to the completion of the wind-up produces a signal to inactivate the motor.
In U.S. Pat. No. 4 383 747, a clutch mechanism is used to transmit the driving power of an electric motor to wind up a photographic film on a spool. This allows the film to be driven frame by frame as the shutter mechanism of the camera is charged ready for the next exposure.
A fully automated camera is disclosed in U.S. Pat. No. 4,506,965 which is capable of a film winding operation during exposure of the film to be changed to film rewinding operation when the end of the film is detected. This is achieved using a one-way coupling mechanism which allows transmission of drive from a winding shaft to a spool during the winding operation, and which disconnects this drive during the rewinding operation. The point at which the drive is disconnected for the rewinding operation is determined by sensing that the film tension has exceeded a predetermined value.
U.S. Pat. No. 4,465,351 describes a motor-operated film driving device in which the driving circuit for the motor can be switched between one of three states, a first state in which the motor is driven forward for film winding, a second state in which the motor is short-circuited and a third state in which the motor is driven in reverse for film rewinding. Detection of a predetermined value of film tension changes the motor from the first to the second state and also generates a delay signal, the motor being changed from the second to the third state in response to the delay signal.
In another arrangement, as described in U.S. Pat. No. 4,419,001, automatic switching between modes is achieved when an increase in motor current is detected on completion of the film wind-up. A member is moved from one position to another on detection of the increased motor current to allow film rewind.
U.S. Pat. No. 4,166,239 discloses another arrangement in which motor current is detected for control of a film drive motor. In this arrangement, a load increase on the motor corresponding to an increase in the tension of the film as its end is reached, in either winding or unwinding modes, causes the motor power supply circuit to be opened stopping movement of the film and thereby preventing damage.
Although it is well-known to used motors to advance and rewind film in cameras as described above, motor current sensing is only used as an indication that the end of the film has been reached, the film being rewound back into its cassette, or that the film is snagging in the camera.